Many of today's communications devices, such as radios, utilize voltage controlled oscillator (VCO) circuits. Phase noise is one of the most important parameters to be considered in the design of oscillator circuits, because it has a great impact on the overall operating performance of the radio. Several techniques exist for optimizing phase noise performance, however, these techniques tend to test/tune the VCO as a stand-alone circuit. Once the VCO is implemented in a transceiver, such as within a phase locked loop (PLL) circuit, the operating characteristics may vary considerably from the previously tuned settings, thus requiring another tuning process. The VCO circuit can end up operating under less than optimal conditions. For some products the feasibility of implementing an additional tuning procedure at the transceiver stage is simply impractical or cost prohibitive.
FIG. 1 shows a graph 100 depicting an example of phase noise 102 vs. current 104 for two given control voltages 106, 108 for a single VCO, such as would operate in a PLL. Curve 106 represents a response for a first control voltage, such as might correspond to channel one of a radio. The optimized phase noise performance for this channel is in the region indicated by designator 110 which corresponds to a particular input current 112. This current now remains fixed across the band. As the radio channel is changed, to channel two for instance, a new control voltage becomes associated with this new channel as indicated by curve 108. The phase noise performance thus varies from the nominal setting indicated by designator 110 to that indicated by designator 114. As seen from the graph 100, considerable phase noise degradation occurs across the band. Graph 100 demonstrates that the VCO can only be truly optimized for a single control voltage, channel, and frequency setting.
Many product lines require additional VCO bandsplits in order to cover a large frequency band. However, these additional bandsplits translate into additional product and manufacturing costs, because separate VCO circuits are needed to cover each band. Alternatively, the total number of VCO bandsplits can be reduced to cover the same total frequency band. This compromise often results in VCOs having reduced performance specifications. It would be desirable to have a single VCO circuit capable of being tuned to operate in more than one bandsplit. In addition, the capability of tuning at the product level as opposed to the stand alone circuit level would further reduce overhead, cost, and time.
Accordingly, there is a need for an improved method and apparatus of optimizing phase noise performance in a VCO circuit.